Block copolymers are macromolecules that include at least two sequences of chemically distinct repeat units. These materials are well known in material science. In a non-limiting example, two homopolymers consisting of repeat units A and B, respectively, are connected by a covalent bond as a diblock copolymer, commonly referred to as an AB diblock copolymer. The average number of repeat units of A and B in a given block copolymer chain can vary, and the average number of A and B repeat units relative to one another may also be varied. In some circumstances, block copolymers may include blocks that contain more than one monomer or have ill-defined or irregular repeat units. Block copolymers can also include triblock copolymers, such as ABA triblock copolymers, and other types of multi-block copolymers that are well known in the art.
Often the two blocks (e.g., including units A and B) may be repulsive or at least more attracted to other blocks of the same kind than to each other, and as a result they may not easily mix with each other. Block incompatibility results in phase separation, the nature of which is dependent on several factors, including the chemical nature of the blocks, the temperature, etc. Because the blocks are covalently attached, phase separation can occur on the nanoscale, resulting in formation of periodic domains of the chemically distinct blocks. The characteristic periodicity of the domain separation is referred to herein as L0. L0 is a term known by those of ordinary skill in the art.
In certain conditions, the block copolymers can undergo phase separation to form periodic nanostructures such as lamellae, cylinders, spheres, etc. as is well known in the art. In thin film applications of block copolymers, the orientation of the block copolymer domains is important. In non-limiting examples, lamellae forming block copolymers in thin films can have their domains oriented parallel or perpendicular to the plane of the substrate upon which they are coated. The orientation of block copolymer domains in thin films is controlled by the chemical nature of the interfaces between the block copolymer domains and the top and bottom surfaces that define the film. For example, if the underlying substrate surface preferentially wets block A, lamellar domains are formed parallel to the plane of the substrate. This creates a stack of parallel domains where block A is in contact with the substrate. In a second example, the underlying substrate may be neutral and does not preferentially wet either block. As a result, the block copolymers can form domains of lamellae, cylinders, etc. perpendicular to the plane of the substrate. Thus, the domains of a lamellar forming block copolymer are oriented perpendicular to the substrate but with no long-range alignment order. In a third example, the underlying substrate may be neutral with defined regions preferential to block A that are about half the length of a periodicity (0.5 L0). Each of these preferential regions attracts the A block of the block copolymer, thereby “pinning” the A domain to that desired area and thereby enabling aligned and oriented vertical domains of blocks. This is known in the art as directed self-assembly. A preferential region about one and a half times the pattern length (1.5 L0) can also be used for pinning a particular domain.
After self-assembly, selective removal of one of the blocks can yield three-dimensional nanoscale relief structures. Selective removal of one block can be achieved via wet or dry etching. For example, blocks A and B etch at different rates under certain reactive ion etch conditions, which allow one block to be selectively removed (referred to herein as etch selective or etch dissipative) and the other to yield structures (referred to herein as etch resistant or etch formative). The etching of such block copolymers that are oriented and aligned as described above can result in a periodic series of deposited lines, pillars, or other structures which may be useful in certain semiconductor or other nanostructure applications.